Separation of fission products,primarily cesium,from uranyl salt solutions by means of an inorganic ion exchanger,zirconium phosphate



United States Patent 3,484,216 SEPARATION OF FISSION PRODUCTS, PRIMARILYCESIUM, FROM URANYL SALT SOLUTIONS BY MEANS OF AN INORGANIC IONEXCHANGER, ZIRCONIUM PHOSPHATE Sten Harald Ahrland, Lund, Sweden,assignor to Aktiebolaget Atomenergi, Stockholm, Sweden, a company ofSweden No Drawing. Filed Feb. 14, 1966, Ser. No. 527,092 Claimspriority, application Sweden, Feb. 23, 1965, 2,340/ 65 Int. Cl. C22b61/04 U.S. Cl. 23-337 7 Claims ABSTRACT OF THE DISCLOSURE Selectiveseparation of ions of alkali (and alkaline earth) metals, such forexample as CS2 and Sr, from acidic aqueous solutions containing suchions in association with uranyl ions is effected by a sorption procedurein which such solution is contacted with a sorbant consisting of aparticulate zirconium phosphate having a controlled mole ratio P:Zr anda controlled content of water of crystallization.

Generally the invention relates to a method of selectively separatingmono-atomic alkali metal and alkaline earth metal ions from themulti-atomic uranyl ion by sorption from a water solution of differentmetal salts, and particularly to a method of separating fission productsfrom uranium in uranyl salt solutions. The separation of cesium andstrontium, having very long-lived radioactive isotopes, is of particularinterest.

In certain modes of processing irradiated fuel elements from nuclearreactors a residual solution is obtained after separating the plutonium,which solution except for the fission products contains practically allthe uranium. From a chemical point of view the soultion is a highlydiluted solution of fission products in a concentrated (-08 .M) uranylnitrate solution, which is, furthermore, relatively strongly (-2 M)acidic from nitric acid. It is desirable to separate the quantitativelysmall content of fission products responsible for practically all of theradioactivity from the solution, primarily in order to enable aconcentration of the activity to a small volume, which to a great extentfacilitates and lowers'the cost for the continued storing thereof.Secondly, there is also aimed at a production in a pure form of fissionproducts interesting from a technical and scientific point of view.

The total amount of fission products is in the order of 2 grams perliter, composed of a very great number of different nuclides. However,only a few of the nuclides essentially contribute to the activity,particularly after the solution having decayed for some period of time,so that all short-lived activities have decayed. In the 10mg run onlythe most long-lived fission products, namely the B-y-emitting 'lCs andthe merely B-emitting Sr, both having a half life of about years, are ofreal importance for the activity of the solution. After five yearsalready these elements are completely dominating; only a smaller part ofthe activity is then due to other nuclides, primarily the pairs Ce- Pr'and Ru-Rh, and Wm. However, at shorter decay times also isotopes ofother elements considerably contribute to the activity, particularly thepairs ZR- Nb and Ba La, and furthermore Y and Rb. The desired separationmethod should be able to separate ions of the types formed from theseelements, and primarily the most important ones, Cs+ and Sr, from thegreat amount of uranyl salt (uranium being present in the form of theion U0 3,484,216 Patented Dec. 16, 1969 ice A method of separationwhich, due to its simplicity in handling, is particularly suitable foractive solutions is that of ion exchange. In view of the relatively highsensitivity to irradiation of the organic ion exchangers only inorganicion exchangers should be taken into consideration for the highly activesolutions involved here.

In view of the limited capacity of the ion exchanger, it is furthermorenecessary to look for a method, in which the separation takes place bysorption of the small amounts of fission product ions on the ionexchanger, while the great amount uranyl ions remain in solution.However, it is difiicult to realize this idea for the reason that withion exchangers, organic as well as inorganic, the tendency to sorb U0 isnormally stronger than, or at least about as strong as that of Cs+ or SrIt has now been found that an ion exchanger of remarka'blecharacteristics can be produced from zirconium phosphate. It has thusbeen found that in a acidic aqueous solution of mono-atomic metal ions,such as of alkali metals, alkali earth metals and rare earths,particularly cesium and strontium, and multi-atomic ions containingmetal, such as the uranyl ion and other actinoic ions of the same type,said ions can be separated by bringing the solution into contact with asolid phase, for instance grains, of zirconium phosphate having a moleratio P:Zr of about 2, preferably within the range 1.8-2.1, particularly1.9-2.0, and having a water content (removable by calcination) of 0.8-2,suitably l-l.5 moles H O per mole Zr. As a rule, optimum effect isobtained at about 1 mole H O per mole Zr, and this optimum is sopronounced, that it is to be suspected that the effective substance iszirconium-monohydrophosphate i.e., the monohydrophosphate group is thefunctional group of the sorbant of the invention. At least with regardto strontium there is a certain effect also below this value. However,the degree of sorption is rather low but the selectivity seems to begood.

The zirconium phosphate should be present in microcrystalline form, forinstance with an average crystallite size of the order of magnitude ofA., i.e. essentially within the range 10-1000 A., for instance 50-500 A.

Sorption experiments with positive ions, particularly with each ofcesium, strontium and uranyl, have been carried out with zirconiumphosphate sorbants of varying water contents obtained by successivedehydration of a preparation relatively rich in water. The experimentshave shown, that at gradually progressing dehydration the sorption of U0soon becomes very slow whereas those of Sr and Cs are influenced verylittle, even if the dehydration is advanced relatively far. Obviously,the interstices of the gel structure,- within which the ions have todiffuse to reach the functional groups, are rather small for an ion asgreat as U0 which has, moreover, a low order of symmetry, and since theinterstices shrink as thedehydration proceeds, the diffusion will verysoon become very much hampered. However, for a smaller ion havingspherical symmetry, such as Sr the interstices are sufficient for arelatively unobstructed diffusion, even if most of the water is removed.Furthermore, it is to be assumed that when the dehydration has beencarried on to a certain critical value, the interstices have shrunk tosuch a degree that the ions no longercan diffuse therein at all. Then arather sudden and very large fall in the load of the gel at equilibriumwill be found, inasmuch as the ions now are restricted to the fewfunctional groups, which are par ticularly easily accessibly positionedon the outer parts of the gel crystallites. At further dehydration thefunctional monohydrophosphate groups will be destroyed. By dehydratingit to an adequate degree it should consequently be possible to obtain agel, which still easily sor-bs Sr and Cs+, but practically completelyexcludes U0 It is not without importance, how the ion exchanger isproduced. For some experiments, which have led to the present invention,the gels used have been produced in the following way: A 0.3 M solutionof zirconyl nitrate in 1 M nitric acid is precipitated with 0.75 Mphosphoric acid solution, until 3 moles phosphoric acid have been addedper mole of zirconium. The formed gel is left in contact with its motherliquor, at frequent stirring, for 2 to 3 days. The phosphate contentthereof, obtained by analyzing the content of phosphoric acid remainingin the mother liquor, then rises from about 1.9 to 2.0-2.1 moles ofphosphate per mole of zirconium. The gel is centrifuged from the motherliquor and dried (under radiator, at 5060 C.) until the loss of weightat calcination to about 1000 C. has decreased to between 50 and 60%.After this preliminary drying, facilitating further washing, the gel isthen seemingly dry and can be crushed to a coarse powder, which iswashed free from still adhering mother liquor with water. The washing iscontinued until the pH of the washwater at equilibrium has increased toabout 3. In view of the fact that the gel is weakly hydrolyzed whentreated with water, phosphoric acid then being released, a higherpH-value is not obtained. The washed gel is dried to a calcination lossof 50%, which is now equal to its water content, and the final ratio ofphosphate/zirconium, P/Zr, is determined by direct analysis. Owing tothe washing this ratio has again decreased; the values found lie between1.89 and 1.98 for different preparations. For those two, the sorptioncharacteristics of which are given below, the values lie between 1.95and 1.98. X-ray diffraction measurements (powder photogram) have shown,that the gels obtained in this way are micro-crystalline, having anaverage crystallite size of about 100 A.

Further dehydration is carried out by drying the gel in an oven atdifferent temperatures until equilibrium is obtained. At eachtemperature the remaining water content is determined by calcination ofsamples taken. The composition of the gel being then known the number ofmoles of water per mole of zirconium can be calculated. By simultaneoussorption experiments with the gels thus treated the selectivity betweenCs+, Sr and U is then determined as a function of moles of water permole of zirconium. The invention is not limited to this method ofproduction, which of course can be modified by those skilled in the art.

As examples of the invention reference will now be made to someexperiments that have been carried out.

In the sorption experiments, for each kind of ion a solution having aninitial concentration of 1 mm. was shaken with the gel, the chosen ratiobetween the volume of solution and the amount of gel being 73.0 ml. pergram of gel, based on the gel less its water content. In this way theused amount of gel, in spite of varying water contents, will alwayscontain the same number of phosphate groups, i.e. the same number offunctional groups, as long as the dehydration has not begun to attackthem. For the gels here used having P/Zr=1.95 and 1.98, respectively,the destruction of the functional groups starts at a water content of0.975 and 0.99 moles H O/mole Zr. Changes of the sorption observed athigher water contents than these must be caused by something else thanthe disappearing of the functional groups.

The sorption effect is judged by measuring the distribution 1, which isdefined as the ratio between the concentration on the gel and theconcentration in the solution after shaking. The distribution isobtained by directly analyzing the solution. Hereinbelow f is given withthedimension l/g.

The selectivity between U0 and Cs+ at pH=1 has been investigated atdifferent shaking times. It is found that cesium and uranyl are sorbedto essentially the same degree, viz., log f=0.6 to -0.4, for watercontents down to about 5 moles H O per mole Zr. After a slight increasein the range of about 5.2 moles H O per mole Zr the sorption of Cs+suddenly increases ever faster, passes a sharp maximum and then fallseven faster down to 0. The very pronounced sorption peak has about thesame height, log 1 about 0.8, for both gels, while the position thereofvaries somewhat. The maximum is for each of the samples at 1.1 and 1.5moles H O/mole Zr for P/Zr=1.95 and 1.98, respectively.

In order that a suitably dehydrated gel shall maintain its highpeak-sorption it is required, that it doesnot rapidly take up water fromthe solution and revert to a condition corresponding to a too high watercontent. It is noted, that the sorption also at the peak decreases onlyslowly with time, and when 2 moles H O/mole Zr has been reached, itdecreases not at all.

At all water contents U0 is sorbed slower than Cs Nevertheless, thesorption occurs fairly rapidly at high water contents but decreasesconsiderably with decreasing water contents. The sorption of UO does notshow any peak at the initial stages of the breaking up of the structure.In striking contrast to what was observed to happen for Cs+, thesorption of U0 here instead begins to decrease strongly.

Thus, there is a relatively narrow range of water contents, wherezirconium phosphate gels of the composition indicated herein above showa very high and selective sorption of Cs+ relative to U0 whereas thesorption of both types of ions is about the same at higher watercontents.

The selectivity between U0 and Sr has been investigated at pH=2, as thesorption of Sr is still very low at the previously used pH=1. ForP/Zr=1.95 and after a short contact time, 4 hours, one finds a curve ofsimilar type as for Cs+, but the peak is here much smaller. At pH=2 U0is sorbed much stronger than is Sr on all gels with a high water contentabove that critical to the invention. However, if the water contentdecreases the sorption of U0 decreases faster and faster, and it becomeslower than for Sr from about 2 moles H O/ mole Zr. Thus, it is true alsofor Sr that at such a low water content it is sorbed considerablystronger by the gel than is U0 in spite of the fact that normally theopposite is the case to a great extent.

A gel having the lower ratio P/Zr=1.22 has not at any water content thestrong preference to Cs+ over UO which is a characterizing feature ofgels with P/Zr close to 2.

The following experiment shows the effect of a simultaneous presence ofcesium and uranyl.

When shaking 147 mg. of zirconium phosphate gel having a ratio P/Zr of1.95 and having 1.08 mole H O/mole Zr (corresponding to 137 mg.waterfree gel) with 10 ml. of a 1 mm. Cs+-solution, which except forcesium nitrate contained only mm. nitric acid (pH=1), Cs+ was sorbed to97.6%.

In an experiment which in all respect was completely identical to theprevious one except for the fact that the solution also held 700 mm.uranyl nitrate, corresponding to 167 g. U/ 1, there was obtained almostas large a sorption of Cs+, 96.5%. Even such a considerable uranylconcentration does thus not significantly influence the sorption of Cs+on a gel treated according to the invention.

The increase of Cs+-sorption caused by the dehydration is of courseimportant for the possibility of its separation from uranium. The factthat the increase is substantial is clear from the fact that a gel asabove having P/Zr=1.95

and containing 8-10 moles H O/mole Zr sorbs only 77% I at the conditionsdescribed above, whereas the gel dehydrated to 1.08 moles H O/mole Zrsorbs 97.6%. The amount of Cs+ remaining in the solution is stantiallydifferent in the both cases.

Experiments carried out with zirconium phosphate preparations producedso as to be given an amorphous structure show that also suchpreparations show selective sorption, the maximum, however, having adifferent charthus sub-' acter. It is broader at least in some cases andstarts already at 2-3 moles H O/ mole Zr.

I claim:

1. A method of selectively separating ions of alkali metals from uranylions, U0 by sorption from an acidic aqueous solution containing suchions, comprising contacting said solution with a sorbant consisting ofsolid particles of zirconium phosphate having a mole ratio P:Zr Withinthe range 1.8-2.1, and having a water content, removable by calcination,of 0.8-2 moles H O per mole Zr.

2. A method as claimed in claim 1, in which the ions selectivelyseparated from said solution are Cs+ ions.

3. A method as claimed in claim 1, in which the mole ratio P:Zr iswithin the range 1.9-2.0, and in which the water content removable bycalcination is within the range 1.0-1.5 moles H O per mole Zr.

4. A method as claimed in claim 1, in which said zirconium phosphate ismicrocrystalline, having a crystallite size substantially within therange of 50-500 A.

5. A method as claimed in claim 1 in which said water content has beenobtained by dehydrating a zirconium phosphate richer in water.

6. A method as claimed in claim 1 for removing fission products of lowconcentration from an acidic concentrated uranyl nitrate solution,comprising contacting the solution with the sorbant defined in claim 1.

7. A method for removing fission products of low concentration from anaqueous acidic concentrated uranyl nitrate solution, comprisingcontacting said solution with solid particles of Zirconium phosphatehaving a mole ratio P:Zr within the range 1.8-2.1 and having a Watercontent, removable by calcination, of 0.8-2 moles H O per mole Zr.

References Cited UNITED STATES PATENTS 2,859,093 11/1958 Russel et a1.23-337 2,970,035 l/ 1961 Stoughton 23337 OTHER REFERENCES I Acta ChemicaScandinavica, vol. 18, 1964, pp. 1357- 1367, Inorganic Ion Exchangers,II Sorption Rate and Dehydration Studies on Zirconium Phosphate andTungstate Gels, S. Ahrland, J. Albertsson, L. Johansson, B. N ihlgard,L. Nilsson.

II Acta Chemica Scandinavica, vol. 18, 1964, pp. 1861- 1878, InorganicIon Exchangers, III Equilibrium Studies on Zirconium Phosphate Gels, S.Ahrland, J. Albertsson.

Amphlett et al., Nuclear Science Abstracts, vol. 18, Abstract #13757(1964).

Harkin et a1., Nuclear Science Abstracts, vol. 18, Abstract #13758(1964).

Healy et al., Nuclear Science Abstracts, Vol. 14, Abstract #248 (1960).

BENJAMIN R. PADGETT, Primary Examiner M. J. McGREAL, Assistant ExaminerUS. Cl. X.R. 23-25; 210-38

